Internal combustion engines may be susceptible to undesired detonation, such as engine knock or pre-ignition, under various conditions. Undesired detonation may cause high in-cylinder pressures and temperatures that can degrade engine components as well as decrease engine efficiency. Various approaches have been developed for identifying and distinguishing pre-ignition and knock, including using acoustic sensors or accelerometers placed on an engine, as well as internal cylinder pressure sensors and ionization sensors. For example, knock and pre-ignition may be identified and distinguished based on differences in timing and intensity when compared to threshold values. Likewise, following detection, distinct steps may be taken for mitigating knock and pre-ignition.
One example approach for addressing knock in displacement on demand (DOD) engines is shown by Stahl et al. in U.S. Pat. No. 6,763,297. Therein, knock detection is performed only on activated cylinders during the deactivated mode of engine operation. Specifically, knock detection is disabled for the deactivated cylinders during the deactivated mode of engine operation to reduce false spark knock detection.
However, the inventors herein have identified potential issues with such an approach. For example, even with the mode-adjusted knock thresholds, cylinder pre-ignition may not be reliably detected or differentiated from cylinder knocking. As such, cylinder pre-ignition may be more likely during the deactivated mode of cylinder operation due to the higher operating load of the running cylinders. Since knock and pre-ignition require substantially different mitigating actions, if cylinder pre-ignition is not reliably detected and appropriately addressed, engine degradation may occur.
Thus in one example, the above issue may be at least partly addressed by a method of operating a boosted engine with selectively deactivatable cylinders. In one example embodiment, the method comprises adjusting a window and a threshold for pre-ignition detection based on a number of deactivated cylinders. The method may further comprise, adjusting a window and a threshold for knock detection based on the number of deactivated cylinders. In this way, by adjusting both the threshold and the window for each of knock and pre-ignition based on the deactivation, pre-ignition may be detected more reliably.
In one example, an engine may include a first group of cylinders with a first downstream exhaust catalyst, and a second group of cylinders with a second downstream exhaust catalyst. An engine control system may select a cylinder group for deactivation based on the regeneration state and temperature of the catalysts, and further based on a deactivation order. Then based on the number of deactivated cylinders in the selected cylinder group, the control system may adjust the window and threshold for each of knock detection and pre-ignition detection. For example, as the number of deactivated cylinders increases, each of the knock and pre-ignition window may be increased to cover a wider range of crank angle degrees. Additionally, the thresholds may be increased. As such, the average knock sensor noise level may decrease as the number of enabled cylinders decreases. Herein, by widening the window when fewer cylinders are enabled, combustion timing differences may be better correlated with abnormal combustion events. Likewise, by increasing the thresholds when fewer cylinders are enabled, knock sensor signal differences may be better correlated with abnormal combustion events. In particular, relatively smaller knock sensor signals related to cylinder knock or pre-ignition may be better differentiated from each other and distinguished from the background noise level.
In this way, incorrect identification of normal combustion events as knocking or pre-ignition may be reduced. By improving the detection and mitigation of abnormal cylinder combustion events, engine fuel economy and efficiency may be increased, while also reducing engine degradation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.